In recent years, image display apparatuses, such as a television receiver, a liquid crystal display, and a plasma display, have different resolutions of image display, and various resolutions have come in. In these image display apparatuses, it is necessary to convert resolutions of signal data, such as a graphics signal independent of a video signal, on-screen data (hereinafter referred to as OSD) for displaying a channel number, volume and the like on a screen, text information multiplexed in a blanking period or the like of a received broadcast and transmitted, and data broadcast information, and to synthesize these signal data with a video signal for display.
A conventional format conversion apparatus performs filtering on a graphics signal to convert resolution of a color component signal, and outputs only the same value of superposition coefficient as that before the resolution conversion (refer to Japanese Published Patent Application No. 2002-271811: Patent Document 1).
Hereinafter, the conventional format conversion apparatus will be described.
FIG. 19 is a block diagram illustrating the outline of the conventional image display apparatus disclosed in Patent Document 1.
With reference to FIG. 19, the conventional image display apparatus comprises a color format converter 1, a resolution converter 2 for a video signal, a color lookup table 3, a resolution converter 11 for a color component signal, a resolution converter 12 for superposition coefficient, and a synthesizer 4. In FIG. 19, the resolution converter 11 for color component signal and the resolution converter 12 for superposition coefficient correspond to the conventional format conversion apparatus.
Hereinafter, a description will be given of the operation of the image display apparatus constructed as described above.
A graphics signal Gindex to be synthesized to an input video signal Vin is supplied in an index format to reduce the amount of data, and color signal components C and superposition coefficients α corresponding to the respective indexes are outputted from the color lookup table 3. The outputted color signal components C are input to the color component signal resolution converter 11, and the superposition coefficients α are input to the superposition coefficient resolution converter 12.
FIG. 20 shows data before and after conversion by the color component signal resolution converter 11 in the conventional format conversion apparatus, wherein the resolution of the color component signal C is converted to ⅔.
With reference to FIG. 20, the color component signal resolution converter 11 converts the inputted color component signals C0, C1, C2, C3, C4, and C5, into C0, (C1+C2)/2, C3, and (C4+C5)/2, whereby the resolutions of the signals are converted. Thereafter, the color component signals are synthesized with a video signal by the synthesizer 4 to output an output video signal Vout.
In the conventional format conversion apparatus, however, when converting resolutions of such as a graphics signal and an OSD signal to be displayed, an averaging process comprising, for example, adding adjacent data to each other and then dividing the result by two, is carried out, whereby a color component signal different from an inputted color component signal is undesirably outputted. Therefore, an unexpected color component appears at the boundary of colors or a medium color is undesirably outputted, whereby image blurring or bleeding occurs when the graphics signal and the OSD signal are displayed, leading to inappropriate display of these signals.
On the other hand, it is also possible to change resolution after superposing an OSD signal or the like on a video signal, using the same format conversion apparatus as that for the video signal. However, in a YUV format for video signal processing, the video signal is processed in a ratio of 4:2:2, while a color difference signal is subjected to time-division processing. Therefore, in an RGB signal format 4:4:4 of an OSD signal or the like, it is necessary to thin out UV signals after YUV conversion to half, resulting in degradation of color resolution, which causes degradation of image quality.
Furthermore, since there is no correlation at the boundary between a video signal and an OSD signal, degradation in image quality might occur during operation processing such as filtering. Further, it might be thought that another format conversion apparatus, which is similar to that for video signal processing, is provided for the OSD signal. In this case, however, the circuit scale is undesirably increased.